A gas turbine, such as an industrial, aircraft or marine gas turbine generally includes, in serial flow order, a compressor, a combustor and a turbine. The turbine has multiple stages with each stage including a row of turbine nozzles and an adjacent row of turbine rotor blades disposed downstream from the turbine nozzles. The turbine nozzles are held stationary within the turbine and the turbine rotor blades rotate with a rotor shaft. The various turbine stages define a hot gas path through the turbine.
During operation, the compressor provides compressed air to the combustor. The compressed air is mixed with fuel and burned in a combustion chamber or reaction zone defined within the combustor to produce a high velocity stream of hot gas. The hot gas flows from the combustor into the hot gas path of the turbine via a turbine inlet. As the hot gas flows through each successive stage, kinetic energy from the high velocity hot gas is transferred to the rows of turbine rotor blades, thus causing the rotor shaft to rotate and produce mechanical work.
Turbine efficiency may be related, at least in part, to the temperature of the hot gas flowing through the turbine hot gas path. For example, the higher the temperature of the hot gas, the greater the overall efficiency of the turbine. The maximum temperature of the hot gas is limited, at least in part, by material properties of the various turbine components such as the turbine nozzles and turbine rotor blades and by the effectiveness of various cooling circuits and a cooling medium that circulates through the cooling circuits to provide cooling to the various turbine components. Turbine nozzles generally include an airfoil that extends in span between an inner band or shroud and an outer band or shroud. The inner band and the outer band define inner and outer flow boundaries of the hot gas path and are exposed to the hot gases. The inner and/or outer bands may be cooled by passing a cooling medium such as compressed air through a central or core cooling channel that extends radially through the airfoil portion of the turbine nozzle. A portion of the cooling medium flows through various film holes defined along the airfoil, thus providing film cooling to the airfoil.